Set parent with automatic weight for selected bones only
Select the vertices in Edit mode, remove them from all vertex groups named like a bone, and add them to this one group that has the same name as the bone you want to parent the vertices to.
Blender creates a vertex group for each bone if you parent the armature With Automatic Weights, With Empty Groups or With Envelope Weights. The weight assigned to the vertices in the group controls the influence of the bone on the mesh.
If you move the hips and the arm moves as well then you need to remove the arm vertices from the hips vertex group. You can do this visually in Weight Paint mode (paint with WP=0), or in Edit mode with plain numbers by editing the values in the N-panel (Item &gt; Vertex Weights) or assigning/removing vertices from the group (Object Properties &gt; Vertex Groups).
If you remove the weight of the hip bone from the arm you should add it to the arm bone (keyword normalize weights).